One well-known hydraulic lash adjuster is described in Japanese Patent Laid-Open No. 61-066811. This lash adjuster includes a cylinder having a base and fixed to a cylinder head, a plunger fitted in the cylinder so as to be movable up and down, and a return spring that powers the plunger in an upwards direction. An upper end part of the plunger, at an end that protrudes from the cylinder, supports a rocker arm. A low-pressure chamber is provided in the plunger, and a high-pressure chamber is provided in a lower end part of the cylinder. The high-pressure and low-pressure chambers are separated by a base wall of the plunger. Operating oil is supplied to the low-pressure chamber via a through hole provided in a circumferential wall of the cylinder, a through hole provided in a circumferential wall of the plunger, and an oil gallery provided in the cylinder head. A valve hole is provided in the base wall of the plunger, and the operating oil flows into the high-pressure chamber from the low-pressure chamber via the valve hole.
In the above-described lash adjuster, a spherical valve body and a return spring are provided in the high-pressure chamber. The return spring powers the valve body in the upwards direction so that the valve body closes the valve hole. Together the valve body and the return spring form a non-return valve for opening and closing the valve hole.
When the rocker arm is pressing down the plunger, the valve hole is closed by the valve body, and the high-pressure chamber is in sealed state. Thus, oil pressure in the operating oil that fills the high-pressure chamber supports the plunger, preventing downwards movement.
When the rocker arm is not pressing down the plunger, the valve body moves downwards, opening the valve hole, and so operating oil flows into the high-pressure chamber from the low-pressure chamber. Thus, the volume of the high-pressure chamber increases, and the plunger is allowed to rise.
In the above-described conventional lash adjuster, the return spring provided in the high-pressure chamber is a compression coil spring. A coil diameter of the compression coil spring is constant along an entire length (from an upper end to a lower end). Hence, a return spring axial direction (the up-down direction) dimension when the plunger is down and the return spring is in a maximally compressed state corresponds to a dimension that is a number windings multiplied by a thickness of the spring wire.
It should be noted that a high-pressure chamber height must accommodate at least the up-down dimension of the return spring when in a maximally compressed state and a plunger stroke length in the up-down direction. Therefore, when the up-down direction dimension of the return spring is large, the height of the high-pressure chamber increases, and an overall height of the lash adjuster has to increase.
The present invention was conceived after considering the above-described problem, and has an object of reducing the height of the lash adjuster.